Photoluminescence represents a mechanism by which a material absorbs electromagnetic energy in the UV-Vis-NIR spectral region at one wavelength, followed by the subsequent emission of a portion of this energy at a different, usually longer wavelength. On the other hand, electroluminescence relies upon the absorption of electrical energy by a material, followed by the emission of visible light. In essence, the absorption of light or electrical energy causes the molecules in the material to typically undergo an electronic transition from typically a ground state to an excited state. Relaxation of the excited-state molecules back to their ground state occurs with the simultaneous emission of light from the material.
The length of time (e.g., delay) between the absorption and emission of light by the material leads to the distinction between the photoluminescence phenomena known as fluorescence and phosphorescence. Typically, the delay time associated with fluorescence is relatively short being on the order of only 10−12 to 10−7 seconds, while the delay for phosphorescence is much longer. In fact, phosphorescent pigments are known to “glow in the dark” releasing the absorbed light over minutes or hours after the light source has been removed.
The occurrence of luminescence can continue to occur with a given photoluminescent or electroluminescent material as long as the external light source or electrical energy is present. If the exciting radiation or energy is stopped, then the occurrence of luminescence will cease. The luminescent effect associated with a material is highly dependent upon the selection of the pigments or luminescent centers in the material, the light absorption properties of the luminescent centers, and the intensity of the light absorbed.
Photoluminescent and electroluminescent materials may be incorporated into a variety of different products and have found use as biological markers for cell imaging, in exit signs and other egress or safety signage, in sensors, for drug delivery, and in optoelectronics, as well as in energy conversion devices. Photoluminescence can be used to track the position of a chemical within the human body and trigger various mechanisms of drug release; such mechanisms including, but not be limited to, microwave, thermal, or photochemical mediated mechanisms. Thus luminescent materials can exhibit multifunctional properties.
New applications and market areas for photoluminescent and electroluminescent materials and devices are continually emerging. Accordingly, there exists a continual desire to develop new materials that are nontoxic, cost effective to manufacture and that exhibit a strong luminescent effect over an extended lifetime. Especially useful is the development of new nontoxic, photoluminescent materials that can be used to deliver a chemical or drug within the human body and electroluminescent materials for optoelectronic displays.